PROJECT SUMMARY/ABSTRACT Malaria has consistently ranked as one of the world's top infectious causes of mortality over the last century. The lack of an effective vaccine reflects a deficiency in our understanding of protective immunity to malaria. This is in large part due to the complex nature of the immune response, which requires both adaptive and innate immune components for successful control and clearance of parasites. Two fundamental questions regarding innate immunity to malaria remain unanswered. First, how are parasites recognized by the immune system? TLR2 and TLR9 have been identified as sensors for malaria infection, but their relevance in vivo remains in question due to the mild phenotypes of infection in Tlr2-/-and Tlr9-/- mice. We have determined that TLR9 is indeed a sensor of early parasite infection in vivo, but that additional unidentified pathways for immune activation exist. These pathways are likely to provide some degree of functional redundancy with TLR9 signaling, which may explain the mildness of the defects observed in Tlr9-/- mice. We also observe that the TLR9 signaling in response to malaria infection is not mediated by the canonical IRF7-dependent pathway. We propose to delineate the components of this unusual TLR9 signaling pathway and to identify the unknown pathway that is redundant with TLR9. Second, it is established that a branch of the innate immune system is critical for control of parasite replication, but the mechanism is not known. Candidates have been proposed, but no study has directly and comprehensively examined the components of the innate immune system that contribute to parasite control. We propose to obtain this information through a systematic and exhaustive in vivo approach, followed by characterization of the leukocyte populations that are determined to be essential for control of the infection. The role of innate immune mechanisms in malaria control is underappreciated, which has likely contributed to our lack of an effective malaria vaccine. This work will address fundamental questions of malaria immunity and lay the groundwork for more informed vaccine design.